The present invention relates to a guide carriage for a linear bearing.
A guide rail for a linear bearing is made known in EP 1 052 480 A1. FIG. 1 of that publication shows a linear bearing, which is composed of a guide rail 3 and a guide carriage 7. The guide carriage is movably supported on the guide rail by four rows of rolling elements 21. To this end, the guide rail includes running surfaces 23 for the rolling elements on its diametrically opposed lateral surfaces. An incremental dimensional standard 29 for a position measuring device is provided on a lateral surface. The dimensional standard is formed by a metal strip, which is provided with a large number of similar, periodically located openings. A sensor 31 for scanning the dimensional standard is provided on the guide carriage.
Reference marks are also provided on the guide rail, in order to specify absolute positions, which serve as starting points for the position determination. The reference marks are designed as separate recesses 41 in the incremental dimensional standard, or as bore holes 41′ in the guide rail. In order to determine the absolute position of the linear roller bearing, the guide carriage must be moved—after the position measuring device has been started up—until the reference sensor detects a reference mark. With each subsequent motion of the guide carriage, the absolute position may be ascertained by adding the change in length—which is determined using the incremental dimensional standard—to the absolute position of the reference mark, with appropriate consideration for the sign.
Publication U.S. Pat. No. 5,563,408 makes known a further position measuring device with an incremental dimensional standard. Instead of the reference marks, a separate, absolute dimensional standard is used, which is located in parallel with the incremental dimensional standard on a single, combined dimensional standard. The absolute dimensional standard is provided with a chain code, i.e., a bit sequence, with which a predetermined number of adjacent bits results in an identification that occurs in only one position of the chain code. A predetermined absolute position of the position measuring device is therefore assigned to each identification. The bits of the identification are read by several adjacent sensors, so that the position measuring device may ascertain its absolute position after start-up, without changing its position.
It should be pointed out that an incremental dimensional standard makes it possible to perform position determination in a substantially more accurate manner than is possible with an absolute dimensional standard, since it uses an interpolation method in combination with the same separation. For example, with an incremental dimensional standard with a separation of 1 mm, the position may be determined exactly up to 0.001 mm. An interpolation method of this type is also disclosed in U.S. Pat. No. 5,563,408. According to FIG. 2, a separation period of the incremental dimensional standard is subdivided into eight parts.
To ensure that the incremental and absolute dimensional standards interact flawlessly, they must have the same separation that is exactly synchronous across the entire length of the dimensional standard, thereby ensuring that an absolute marking is unequivocally assigned to every incremental marking. This exact synchronization may only be attained, in a cost-favorable manner, when the absolute and incremental dimensional standards are designed as single pieces, as a combined dimensional standard.